The first known living sample of a giant, ancient mollusc that previously was known almost exclusively by its shells has been recovered from the Philippines.

A team of researchers have finally come across a live colony of giant shipworms, or Kuphus polythalamia. Washed-up, empty, elephant tusk-like shells first hinted at the existence of this metre-long animal in the 18th century, and there are a few specimens preserved in ethanol in collections around the world. But no one knew exactly what lay within – until now.

Daniel Distel at the Ocean Genome Legacy Center at Northeastern University in Boston and his colleagues were made aware of the animal’s potential location in 2010, when a collaborator pointed out a news story from Philippine TV featuring a local trying to eat one for its supposed medicinal properties. “[It was] amazing! I’ve been looking for them for 20 years,” Distel says. “My friend and mentor Ruth Turner looked for her whole career.”

The TV footage sparked an international search for the giant shipworm. Local researchers embarked on two expeditions, one in 2010 and one in 2011. During the second expedition, they found live specimens of K. polythalamia and transported them to the University of the Philippines to be analysed.

“It’s hard not to be amazed when seeing one in the flesh, even if you know nothing about them,” Distel says. “There is no other animal like them.”

NOXIOUS MUD

The giant shipworm is the world’s longest bivalve and the only known extant species of its genus. Unlike other shipworm species that typically burrow into and eat driftwood, the giant shipworm inhabits organic rich, noxious mud and doesn’t seem to feed much at all. Instead it relies on symbiotic bacteria in its gills.

“Bacterial symbionts have been cropping up in various bivalve families,” says John Taylor at London’s Natural History Museum, who was not a member of the expedition. “It’s only since the early 1980s that we recognised that bivalves have bacterial symbionts at all.” Finding them in the family that K. polythalamia belong to as well expands this relationship.

The bacteria break down the mud’s hydrogen sulfide, which give the shipworm’s habitat a distinct rotten egg smell, and produce organic carbon, feeding their host. The shipworm is so reliant on the bacteria that many of its internal digestive organs have shrank from lack of use.

NEW ADDITION

This isn’t the only surprise harboured in the giant’s shell. It turns out the giant shipworm is a newer addition to the family tree, rather than an ancient ancestor.

“Kuphus was thought to be the most primitive shipworm,” Distel says. “Instead it turns out to have evolved from wood eating shipworms. Its anatomical simplicity is actually the result of specialisation for a new lifestyle rather than the lack thereof.”

Though Distel’s team presented their initial findings in 2013, they are only releasing their full results now after vigorous testing, including extensive genome sequencing and the first ever culture of a sulfur oxidizing symbiont.

“This is one of those cases where extraordinary claims required extraordinary evidence – and we really hammered this one,” Distel says.